Recently, a periplasmic protein from the sulfate-reducing bacterium, Desulfovibrio vulgaris, was found to contain approximately equal numbers of both rubredoxin-like and hemerythrin-like iron atoms. This protein has therefore been dubbed rubrerythrin. Detailed physical and chemical characterizations of the iron sites in rubrerythrin are proposed. The goals of the proposed studies are the delineation of a function for rubrerythrin in the metabolism of D. vulgaris, and descriptions of the structures and reactivities of the iron sites in terms of this function. Working hypotheses are that rubrerythrin plays a specific role in the sulfur metabolism of D. vulgaris, and that its hemerythrin-like binuclear iron site interacts directly with the sulfur metabolites. Reactions between rubrerythrin and another periplasmic protein, such as cytochrome c553, may be required in vivo. Based on these hypotheses, examinations of the structural and magnetic properties of the two iron sites in rubrerythrin by 1H NMR, EPR, and resonance Raman spectroscopies are proposed. Comparisons of these spectra will be made with those of the analogous prototype iron sites. Proposed chemical studies include indentifications and quantitations of the products of oxidations of sulfide and sulfite by rubrerythrin, and by the combination of rubrerythrin and cytochrome c553. Thorough examinations of the reduction potentials of the iron sites in rubrerythrin are proposed, as are interactions and redox reactions between rubrerythrin and cytochrome c553. These studies are to be complemented by amino acid sequence analysis and x-ray crystallography of rubrerythrin. The dissimilatory reduction of sulfate to sulfide by bacteria of the genus, Desulfovibrio, plays an important role in partially anoxic ecosystems, and the discovery of rubrerythrin may mean that a unique combination of non-heme iron sites catalyzes crucial chemical transformations in this sulfur cycle. This project represents an excellent opportunity to apply modern physical methods and inorganic chemistry to gain insight into an important biological process.